An RNA virus population does not consist of a single genotype; rather, it is an ensemble of related sequences, termed quasispecies. Quasispecies arise from rapid genomic evolution powered by the high mutation rate of RNA viral replication. Although a high mutation rate is dangerous for a virus because it results in nonviable individuals, it has been hypothesized that high mutation rates create a 'cloud' of potentially beneficial mutations at the population level, which afford the viral quasispecies a greater probability to evolve and adapt to new environments and challenges during infection. Mathematical models predict that viral quasispecies are not simply a collection of diverse mutants but a group of interactive variants, which together contribute to the characteristics of the population. According to this view, viral populations, rather than individual variants, are the target of evolutionary selection. Here we test this hypothesis by examining the consequences of limiting genomic diversity on viral populations. We find that poliovirus carrying a high-fidelity polymerase replicates at wild-type levels but generates less genomic diversity and is unable to adapt to adverse growth conditions. In infected animals, the reduced viral diversity leads to loss of neurotropism and an attenuated pathogenic phenotype. Notably, using chemical mutagenesis to expand quasispecies diversity of the high-fidelity virus before infection restores neurotropism and pathogenesis. Analysis of viruses isolated from brain provides direct evidence for complementation between members in the quasispecies, indicating that selection indeed occurs at the population level rather than on individual variants. Our study provides direct evidence for a fundamental prediction of the quasispecies theory and establishes a link between mutation rate, population dynamics and pathogenesis.

respectively (20). I11 the case of two-photon absorption, the line-shape f~~n c t i o r l(21 ), where n is the transition frequency, v is the laser frequency, and r is the molec-SCIENCE VOL. 271 22 MARCH 1996 22. C. SchGnenberger, J. Jonitsma, J. A M. Sondag-Huethorst. L G. J. Fokkink, J. Phys. CChem. 99.3259(1 995). 23. We thank W. Emst for the loan of a microwave source.

The ribonucleoside analog ribavirin (1-beta-D-ribofuranosyl-1,2, 4-triazole-3-carboxamide) shows antiviral activity against a variety of RNA viruses and is used in combination with interferon-alpha to treat hepatitis C virus infection. Here we show in vitro use of ribavirin triphosphate by a model viral RNA polymerase, poliovirus 3Dpol. Ribavirin incorporation is mutagenic, as it templates incorporation of cytidine and uridine with equal efficiency. Ribavirin reduces infectious poliovirus production to as little as 0. 00001% in cell culture. The antiviral activity of ribavirin correlates directly with its mutagenic activity. These data indicate that ribavirin forces the virus into 'error catastrophe'. Thus, mutagenic ribonucleosides may represent an important class of anti-RNA virus agents.

Electron transfer through molecular frameworks is central to a wide range of chemical, physical, and biological processes. We demonstrate a means to measure electronically and to quantify electron transfer through organic molecules and films. We show quantitative agreement with universal values of electron transfer inferred from biological, electrochemical, photochemical, and related systems. Scanning tunneling microscopy was used to image adjacent chains and molecular terraces of different length alkanethiolates in an ordered selfassembled monolayer lattice on Au{111}. In electron transfer measurements using a scanning tunneling microscope, both the driving force and the electrode separation can be continuously varied. This allows independent electronic measurement of the molecular bridges through which electron transfer takes place. The differences between the measured topography in scanning tunneling microscopy and the physical heights of these molecules can be understood in terms of the transconductance through individual chains using a two-layer tunnel junction model.

Alkanethiolate self-assembled monolayers on Au{111} are used for the two-dimensional matrix isolation of conjugated organic thiolates, studied for their potential to act as molecular wires. The conjugated organic molecules are inserted from dilute solution into boundaries between structural domains and at substrate step edges of preassembled n-alkanethiolate monolayers so as to preserve the order of the initial alkanethiolate lattice. In contrast, when both molecules are codeposited from a single solution, the structure of the assembled monolayer shows no ordered molecular lattice of the alkanethiol component. Scanning tunneling microscopy measurements on the isolated conjugated component molecules show them to have enhanced conductivity compared to neighboring alkanethiolates.